[Haskell-cafe] groupBy huh?
wren ng thornton
wren at freegeek.org
Fri Mar 4 06:54:53 CET 2011
On 3/3/11 7:18 PM, Jacek Generowicz wrote:
> Hi Cafe,
> It seems that I don't understand what groupBy does.
> I expect it to group together elements as long as adjacent ones satisfy
> the predicate, so I would expect ALL four of the following to give one
> group of 3 and a group of 1.
> Prelude> :m + Data.List
> Prelude Data.List> groupBy (<) "abcb"
> Prelude Data.List> groupBy (<) "abca"
> Prelude Data.List> groupBy (<) [1,2,3,2]
> Prelude Data.List> groupBy (<) [1,2,3,1]
> What am I missing?
The behavior is that it's comparing subsequent elements to the first
element of the current chunk. I'm not sure how often that'd be a
desirable behavior compared to the one you and I would expect. Of
course, the API only specifies the behavior of groupBy on equality-like
predicates IIRC. So technically either behavior is permissible...
This should be FAQed on the documentation a bit better.
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